Increasing importance is being attached to the cold-setting polyurethane-based binders in connection with binder-containing moulding material mixtures for producing moulds and cores. These binders generally result from the reaction of polyisocyanates with at least 2 NCO groups and polyols with at least 2 OH groups.
In the foundry industry, the polyurethane binder is most frequently used in the form of a three-component system. For this purpose, generally the granular mould base material (e.g. quartz sand or the like) is initially mixed with the polyol (e.g. a phenol resin having a benzyl ether-like construction) and then with the polyisocyanate and subsequently reaction accelerators (e.g. tertiary amines or organometallic compounds) are added so that the binder sets in a sufficiently short time. This addition of reaction accelerators can take place in the final phase of the mixing process or after the moulding material mixture has been removed from the mould by briefly gasing the mixture in the mould with a gaseous tertiary amine such as e.g. triethyl amine (gas hardening process).
All known three-component systems have the disadvantage that the moulded blanks only relatively slowly acquire their final strength or at least a strength adequate for the production of high quality castings, so that it is necessary to intermediately store them for a relatively long time after removal from the mould.
The gas hardening process has the additional disadvantage that an additional process stage is involved, which is very costly from the apparatus standpoint, so that workers do not suffer from the toxic and extremely evil-smelling amines.
Belgian Pat. No. 865,741 already discloses a polyurethane binder for foundry purposes, which is used as a two-component system. In this binder, the polyol is an aminopolyol which, in addition to its OH groups, contains at least one tertiary amino group in the molecule which acts as an accelerator. As a result of this special molecule structure, the binder can be set or hardened very rapidly and uniformly without it being necessary to add additional accelerators. As a result, it is possible to use the moulded blanks after they have been stored for a much shorter time and the disadvantages of the known three-component systems are completely eliminated.
In the two-component binders known from Belgian Pat. No. 865,741, the aminopolyol is produced by reacting alkylene oxides with polyvalent primary or secondary aliphatic amines. These binders have proved eminently suitable, e.g. for light metal castings. However, it has been proved disadvantageous that they are less suitable for metals with a very high casting temperature such as e.g. grey cast iron, steel, etc. Thus, they have a somewhat thermoplastic nature and consequently do not have an optimum thermal stability, so that casting defects can occur at higher casting temperatures above about 1000.degree. C. As a result of working with alkylene oxides, their production is correspondingly costly and difficult.
It is the object of the present invention to provide a cold-setting polyurethane binder representing a two-component system with an aminopolyol as one component which combines the advantage attainable with this system of a rapid possibility of using the moulded blanks, with a considerably improved thermal stability.